Generally, a fuel cell is an electric power generation system that converts chemical energy into electrical energy based on a chemical reaction between hydrogen from a hydrocarbon-based material such as, for example, methanol, ethanol, and natural gas, and oxygen.
Depending on the type of electrolyte used, fuel cells are divided into categories including a Phosphoric Acid Fuel Cell (PAFC) that operates at around 150 to 200° C., a Molten Carbonate Fuel Cell (MCFC) that operates at a high temperature of 600 to 700° C., a Solid Oxide Fuel Cell (SOFC) that operates at a high temperature of over 1000° C., and a Proton Exchange Membrane Fuel Cell (PEMFC) and a Alkaline Fuel Cell (AFC) that operate between room temperature and 100° C. These fuel cells all operate on the same principle, but the types of fuel, operating temperature, catalyst, and the electrolyte used are different from each other.
The PEMFC has excellent output characteristics, fast starting and response characteristics, as well as a low operating temperature, when compared to other types of fuel cells. It also has the advantage of having a wide range of applications including use as a distributed power source for houses and large buildings, as a small power source for electronic devices, and as a mobile power source for a car.
The basic structure of a PEMFC system includes a fuel cell body called a stack, a fuel tank, and a fuel pump that supplies fuel from the fuel tank to the stack. It further requires a reformer that generates hydrogen by converting fuel such as methanol, ethanol, or natural gas from the fuel tank and supplies the hydrogen to the stack. The PEMFC generates electricity by first pumping the fuel stored in the fuel tank to the reformer using the fuel pump. Then hydrogen gas is generated through the fuel reformation in the reformer and subsequently reacts with oxygen in the stack.
In addition, the fuel cell can have a Direct Methanol Fuel Cell (DMFC) configuration in which liquid-phase methanol fuel is directly supplied to the stack. Unlike PEMFCs, DMFCs do not require a reformer.
In the above fuel cell system, the stack includes numerous unit cells that are arranged in multiple layers. Each unit cell is formed of a membrane-electrode assembly (MEA) and a separator or bipolar plate. The membrane-electrode assembly includes an anode and a cathode that are each attached to either side of an electrolyte membrane. The separator provides a path for supplying hydrogen gas and oxygen and serves as a conductor that couples the anode and cathode of the membrane-electrode assembly. The separator allows hydrogen gas to be supplied to the anode and for oxygen to be supplied to the cathode. The hydrogen gas undergoes an electrochemical oxidation reaction at the anode and the oxygen undergoes an electrochemical reduction reaction at the cathode. The transfer of electrons during the reactions generates electricity as well as heat and water.
A platinum (Pt) catalyst is usually used for the oxygen reduction reaction at the cathode. However, since platinum is expensive, metal alloy catalysts have been researched.
U.S. Pat. No. 4,447,506 discloses metal alloy catalysts that include platinum-chromium-cobalt (Pt—Cr—Co), platinum-chromium (Pt—Cr), and the like. Also, U.S. Pat. No. 4,822,699 discloses metal alloy catalysts such as platinum-gallium (Pt—Ga) and platinum-chromium (Pt—Cr). However, none of the patent literature provides a clear explanation of how the binding force of the platinum-oxygen bond affects the oxidation and reduction reaction mechanisms and the overall activity of a catalyst.